1. Field of the Invention
The present invention relates to the composition of a hot working powdered lubricant most suitable for Mandrel Mill rolling of seamless tubes and pipe (hereinafter, referred to as simply a “tube(s)”) and a method for manufacturing seamless tubes by applying the same, and more particularly, to the composition of the hot working powdered lubricant, which enables to improve the quality of inside surface of finished-product tubes in Mandrel Mill rolling, and the method for manufacturing seamless tubes by applying the same.
2. Description of the Related Art
In steps of manufacturing seamless tubes by Mandrel Mill rolling, a round billet as a starting material is heated in a heating furnace, and then is subjected to a piercing-elongation rolling process to thereby obtain a hollow tube stock (also, referred to-as “hollow shell”), which is to be rolled subsequently. Next, a mandrel bar is inserted into the bore of the pierced hollow tube stock that is held at high temperatures in the range of 1000 to 1300° C. and an elongation-rolling process is applied to obtain a tube blank in the Mandrel Mill consisting of seven or eight pairs of grooved rolls in tandem wherein each pair thereof is out of phase by 90-degree to one another. Then after that, the tube blank thus obtained is reheated by a reheating furnace, when needed, and subjected to a finishing rolling by Stretch Reducer Mill to obtain finished-product tubes with predetermined dimensions.
When the hollow tube stock is subjected to the elongation-rolling process, relative sliding motion occurs at the interacting surface between the inside surface of the hollow tube stock and the outer surface of the mandrel bar. In Mandrel Mill rolling, it is a key technological issue to maintain the interacting surface in a good lubricating condition so as to smoothly let the relative sliding motion occur. To that end, a lubricant is usually applied on an interacting surface to secure a low and stable friction co-efficient, thereby enabling to prevent seizures between the hollow tube stock and the mandrel bar, while obtaining a good quality of inside surface of and good dimensional accuracy of finished-product tubes.
As the lubrication method above, a method for forming a solid lubrication film is applied in such a way that a water-dispersed type lubricant mainly consisting of graphite and resin-type organic binder is coated and dried over the surface of mandrel bar prior to be inserted into a hollow tube stock.
Meanwhile, as the lubricant to be used rendering it attached on the working surface of hollow tube stock, various lubricants have been studied in succession thus far, and any one is yet to exhibit sufficient effect. In particular, a technological improvement in Mandel Mill rolling is remarkably made, so that in Retained Mandrel Mill rolling, a required friction co-efficient of lubricant and quality of inside surface of finished-product tubes are not fully met yet.
Further, in Full Retract Mandrel Mill that recently becomes popular, a shorter mandrel bar is adopted in subjecting a longer length tube blank to an elongation-rolling process, so that the reduction of the friction co-efficient becomes essential and conventional lubricants put heavier burden on the mandrel bar, wherein seizures are very likely to be generated to lower the quality of the inside surface.
In this regard, Japanese Patent Application Publication No. 2002-338984 proposes the composition of powdered lubricants for hot working wherein by virtue of optimizing physical properties thereof in a powder state so as to be homogeneously sprayed to the predetermined working positions when sprayed into the inside surface of hollow tube stock, the reduction of friction between a hollow tube stock and a mandrel bar can be achieved, which is made up of a primary component including sodium borate in pentahydrate and an auxiliary lubricant including sodium carbonate and the like.
When the powdered lubricants proposed in Patent Application Publication above are used in Mandrel Mill rolling, not only operability can be improved because they are easily handled, but also the friction between the hollow tube stock and the mandrel bar during rolling can greatly be reduced, whereby inside-surface defects to be generated for finished-product tubes can be suppressed.
On the other hand, as a surface layer of a Cr-plated mandrel bar is typically covered with chromium oxide and passivated, corrosion hardly occurs, but when brought into contact with substances such as sodium borate that dissolve metal oxide at high temperatures, the chromium oxide on the Cr-plated surface may happen to be dissolved to result in a kind of corrosion-wear.
To cope with this, Japanese Patent Application Publication No. 2002-338985 proposes the composition of powdered lubricants for hot working which comprise sodium borate and the like, wherein a corrosion damage of a Cr layer on the surface layer of mandrel bar can be suppressed, while enabling to extend a tool life. When these powdered lubricants are used in Mandrel Mill rolling, not only the corrosion-wear on the Cr-plated surface can be suppressed but also the span of life of hot working tool can be prolonged, while the stable quality of inside surface of finished-product tubes can be secured.